Process for glycol esters of an alkali metal salt of 5-sulfoisophthalic acid

ABSTRACT

A process for preparing bis alkylene glycol esters of an alkali metal salt of 5-sulfoisophthalic acid. The subject compound is useful in preparing modified polyesters which can be dyed with basic or cationic dyes. The process is economical and low cost since intermediate reaction products are not purified.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention is direct to the field of monomers for cationic dyeablepolyesters.

2. Background

Polyester fibers based on the polyesterification of terephthalic acidand alkylene glycols are well known. Polyester fibers are importanttextile fibers but they are difficult to dye by normal dyeing techniquesand are generally dyed using disperse dyes. However, polyester fiberscan be rendered dyeable by basic and acid dyes by incorporating amodifying monomer into the polyester chain.

Suitable monomers for rendering a polyester dyeable by a cationic orbasic dyes are the bis glycol esters of the sodium salt of5-sulfoisophthalic acid. U.S. Pat. No. 3,936,389 describes the use ofsulfonated monomers in preparing cationic dyeable polyester fibers anddiscloses that the preferred sulfonate is a bis ethylene glycol ester ofthe sodium salt of 5-sulfoisophthalic acid. The glycol ester is preparedfrom the dimethyl ester of the sodium salt of 5-sulfoisophthalic acidand ethylene glycol at 160° C. to 250° C. using an ester interchangecatalyst; e.g. lithium acetate and calcium acetate.

U.S. Pat. No. 3,018,272 discloses the preparation of potassium 3,5di(carbomethoxy) benzenesulfonate from sulfoisophthalic acid in amethanol-benzene solvent mixture. The methyl ester is used intransesterification reaction to prepare a cationic dyeable polyester.

The sodium salt of 5-sulfoisophthalic acid is commercially available andit may be used to prepare its bis glycol esters. However, thecommercially available sodium salt is expensive, and attempts to preparethe bis ester by simple esterification proved unsuccessful.Additionally, this route is economically unattractive because of thecost of the commercially available acid. Commercially available bisglycol esters are prepared by transesterification using the dimethylester as a starting material. Furthermore, commercially available bisethylene glycol esters of the sodium salt of 5-sulfoisophthalate are notpure compounds but rather mixtures containing about 60 to 75 percent ofthe bis ethylene glycol ester, about 20 to 25 percent of the mixedethylene glycol / methyl ester and about 3 to 6 percent of the dimethylester. Under polymerization conditions methanol is split off from themixed and dimethyl esters. The low boiling methanol causes processingand safety problems at the high polymerization and extrusiontemperatures. The methanol must be removed from the polymer melt withoutdischarge to the atmosphere which presents an additional environmentalproblem. It is estimated that methanol recovery from a directesterification polyester production line could cost millions of dollarsper year.

It is the object of this invention to provide a process for preparingmethanol-free, bis glycol esters of 5-sulfoisophthalic acid alkali metalsalts in high purity from isophthalic acid in a most economical way.

SUMMARY OF THE INVENTION

The invention is a process for preparing methanol-free, bis alkyleneglycol esters of the alkali metal salts of 5-sulfoisophthalic acid inhigh purity at low cost from isophthalic acid without the purificationof intermediate products. These glycol esters have the following generalformnula: ##STR1## where: n is an integer independently selected from 1to 10, preferably 1 to 3, and M is alkali metal moiety. The integer n ismost preferably 1 and M is most preferably sodium. The alkylene glycolmoiety may be either linear or branched if n is greater than 2, mostpreferably linear.

The process of the invention comprises sulfonating isophthalic acid andneutralizing the sulfo moiety with an alkali metal neutralizing agent,recovering the reaction products of the sulfonation reaction withoutpurification, dispersing the reaction products of the sulfonationreaction in an alkylene glycol solvent; adjusting the pH of the reactionmixture, and esterifying the carboxylic acid groups of the5-sulfoisophthalic acid alkali metal salt with said alkylene glycolsolvent in the presence of a catalyst or without a catalyst. The bisalkylene glycol ester of the 5-sulfoisophthalic acid mono-alkali metalsalt is separated from the alkylene glycol insoluble reaction productsto provide a solution of bis glycol ester in glycol.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

This invention is that of a high yield, cost effective process forproducing a methanol-free, alkali salts of a bis alkylene glycol esterof 5-sulfoisophthalic acid from isophthalic acid. The preferred salt isthe monosodium salt of the bis ethylene glycol ester of5-sulfoisophthalic acid which is useful in the preparation of cationicdyeable polyester substrates.

The alkali salts of bis alkylene glycol esters of 5-sulfoisophthalicacid which may be prepared according to the process of the inventionhave the following general formula: ##STR2## wherein: n is an integerindependently selected from 1 to 10 and M is an alkali selected fromlithium, sodium, potassium, and cesium. In the above formula n ispreferably 1 to 3, most preferably 1 and M is preferably sodium orpotassium, most preferably sodium. The alkylene glycol moiety may beeither linear or branched if n is greater than 2, most preferablylinear.

The invention provides a low cost, methanol-free, bis glycol esterbecause methanol based intermediates are not used and purification ofintermediate reaction products is eliminated. Theoretically,commercially available sodium salt of 5-sulfoisophthalic acid can beused to prepare methanol-free bis alkylene glycol esters. However thiscommercially available starting material is expensive since it ispurified after sulfonation to remove reaction by-products such as sodiumsulfate, sulfuric acid, water, etc. Additionally, very low yields wereobtained by reacting ethylene glycol with the commercially availableacid. Surprisingly, we have found that the insoluble, wet reactionproducts of the sulfonation and neutralization steps may be dispersed ina glycol solvent and the salt of the 5-sulfoisophthalic acid isesterifled without purification. The esterified alkali metal salt of5-sulfoisophthalic acid is then separated from the insoluble sulfonationreaction products to provide a solution of bis glycol ester in glycol.

The process of the invention comprises the following steps:

(a) sulfonating isophthalic acid at the 5-ring position;

(b) neutralizing the 5-ring position sulfo group with an alkali metalneutralizing agent;

(c) recovering the neutralized reaction products of step (b);

(d) dispersing said neutralized reaction products in a polyalkyleneglycol;

(c) adjusting the pH of said dispersion to about 1.7 to 5;

(f) heating said dispersion to remove water and to form a bis glycolester of said neutralized 5-sulfoisophthalic acid with or withoutcatalyst and

(g) separating said bis glycol ester from insoluble reaction products.

The sulfonation of isophthalic acid can be conducted according tomethods well known in the art. Preferably, the sulfonation is conductedin oleum at a temperature from about 125° to 200° C., preferably fromabout 140° to 175° C. and most preferably about 150° to 160° C. Therequired reaction time will vary from 2 to 24 hours depending upon thereaction temperature and excess of sulfur trioxide. Generally 5 to 15hours at 150° to 160° C. will produce satisfactory results.

The sulfonation is usually carried out in oleum, a mixture ofconcentrated sulfuric acid and sulfur trioxide. The reaction is carriedout both with the commercially available oleum or by mixing concentratedsulfuric acid with sulfur trioxide during the reaction. Isophthalic acidand sulfuric acid are employed in a mole ratio ranging from about 1:1 toabout 1:10, respectively with the preferred mole ratio being between 1:2to 1:5. Although, one mole of sulfur trioxide is required to sulfonateone mole of the isophthalic acid to produce the product, it is preferredthat an excess amount of sulfur trioxide be used in the reaction.Accordingly, the amount of sulfur trioxide should be in the range of 1.2to 3 moles of sulfur trioxide per mole of isophthalic acid, preferablyabout 1.5 to 2. Alkali metal sulfates such as sodium sulfate orpotassium sulfate may be added to the sulfonation reaction to improvequality and to minimize by-products. The amount of additives is in therange of mole ratios from about 1:0.01 to about 1:0.1.

After completing the sulfonation, the reaction mixture is cooled to roomtemperature and drown into an ice/water mixture at a temperature ofabout 0° to 80° C., preferably about 0° to 40° C., and most preferablyabout 0° to 20° C. The 5-sulfo ring substituent is neutralized using analkali metal neutralizing agent such as lithium hydroxide, sodiumhydroxide, potassium hydroxide, sodium carbonate, cesium hydroxide,sodium bicarbonate etc. The neutralizing agent is preferably a sodium orpotassium base; most preferably sodium hydroxide or sodium carbonate.Approximately two equivalents of base are required to achieveneutralization of the 5-sulfo substituent due to the highly acidicnature of the reaction medium.

The neutralized 5-sulfoisophthalic acid salt will precipitate from theaqueous reaction medium. Preferably the reaction medium should betreated with salt such as sodium sulfate or sodium chloride to ensurecomplete precipitation. The precipitated product is then recovered byconventional methods such as filtration or centrifugation. The wetprecipitate should not be washed due to the high solubility of thealkali metal salt of 5-sulfoisophthalic acid. When recovered byfiltration, the filter cake contains about 2 to 10 percent inorganicsalt (M₂ SO₄), about 5-15 percent water, 2 to 5 percent sulfuric acidand about 65 to 90% percent of the alkali metal salt of5-sulfoisophthalic acid product, wherein M is defined above.

The wet, acidic precipitate is esterified by slurrying or dispersing itin the desired alkylene glycol. The glycol employed is of the generalformula HO-CH₂ -(CH₂)_(n) -OH, where n is an integer from 1 to 10. Apreferred glycol is ethylene or propylene glycol, most preferablyethylene glycol. Optionally a mixture of glycols may be used. The slurryshould contain sufficient glycol to maintain fluidity and esterify thecarboxylic acid functions of the 5-sulfoisophthalic acid salt. Thealkali metal salt of 5-sulfoisophthalic acid and glycol are employed ina mole ratio ranging from about 1:2 to about 1:20 with the preferredmole ratio being 1:7 to 1:14.

The pH of the slurry is adjusted with a base such as sodium hydroxide orsodium carbonate to neutralize free acid in the slurry. In order toobtain an accurate pH measurement, it is desirable to dilute the pH testspecimen with one part of distilled water per part of specimen prior topH measurement. A relative pH of 1.7 to 5 provides satisfactory results;preferably the pH should be about 2 to 3. At a relative pH below 1.7,the reaction does not proceed in a satisfactory manner, i.e. the processgives erratic results--low yield and/or large amounts of undesiredby-products.

The reaction mixture is heated to drive off residual water and after thewater is removed the reaction mixture is heated to esterify the5-sulfoisophthalic acid salt. Generally a temperature from about 155° to220°0 C., preferably 170° to 200° C. gives acceptable results withremoval of the water of reaction to drive the esterification tocompletion. The elevated temperature is maintained until the reaction issubstantially completed as indicated by the cessation of the evolutionof the water of reaction.

The esterification is carried out with or without a catalyst, preferablywithout a catalyst. Conventional esterification catalyst may beemployed, such as an acetate salt of metals such as sodium acetate,calcium acetate, manganese acetate. If a catalyst is employed, thealkali metal salt of 5-sulfoisophthalic acid and catalyst are employedin a mole ratio ranging from about 20:1 to about 1000:1, preferably 50:1to 200:1.

The bis glycol ester of the 5-sulfoisophthalic acid salt is soluble inthe glycol and can be recovered from the insoluble inorganic saltsproduced in neutralizing the sulfonation medium. The inorganicsalt-glycol-bis glycol ester mixture is viscous slurry at roomtemperature and it should be heated to improve the filtration orcentrifugation to obtain the solution of bis glycol esters of the alkalimetal salts of 5-sulfoisophthalic acid.

The following examples illustrate the invention.

EXAMPLE 1 Sulfonation of Isophthalic Acid with 30% Oleum

A 500 mL three neck flask fitted with a mechanical stirrer, thermometer,and a reflux condenser with dry tube was charged with 118.2 mL of 30%oleum, 6.0 g of sodium sulfate, and 83.0 g of isophthalic acid. Thereaction mixture was heated to 155° C. and held at this temperature for11 hours. The mixture was cooled to 25° C. and drowned into 500 g of iceand water. The solution was kept at 0°-20° C. with external coolingwhile 118.0 g of 50% sodium hydroxide was added. The precipitatedproduct was isolated by filtration to give 147.5 g of material whichcontained 115.5 g (86% yield) of 5-sulfoisophthalic acid sodium salt.

EXAMPLE 2 Sulfonation of Isophthalic Acid with Sulfur Trioxide

A 300 mL autoclave was charged with 67.9 g of sulfuric acid 100%, 2.3 gof sodium sulfate and 33.2 g of isophthalic acid. The reactor was sealedand 29.1 g of sulfur trioxide was added to the reaction. The reactionmixture was heated to 155° C. and held at this temperature for 11 hours.The mixture was cooled to 85° C. and drowned into 220.0 g of ice andwater. The solution was kept at 15°-20° C. with external cooling while33.0 g of 50% sodium hydroxide was added. The product precipitated as athick slurry. To complete the precipitation, 20 g of sodium sulfate wasadded. The product was isolated by filtration and 50.0 g of wet solidswere collected which were found to contain 45.6 g of 5-sulfoisophthalicacid sodium salt.

EXAMPLE 3 Esterification of 5-Sulfoisophthalic acid Sodium Salt (SIPA)at pH 0.4

A 500 mL round bottom flask was charged with 300 mL ethylene glycol and153.3 g of wet acidic cake containing 134.1 g of 5-sulfoisophthalic acidsodium salt. The pH of the slurry was found to be 0.4. About 0.8 g ofmanganese acetate was added and the reaction heated to reflux. Thedistillate was removed from the reaction mixture as the temperature roseto about 110° C. The temperature of the reaction did not rise above 140°C. The distillation was maintained for several hours until the solidproduct began to build up around the stirrer. The reaction mixture wascooled to room temperature and analyzed by HPLC. The HPLC analysis foundthat less than 2% product was formed. The distillate contained a largeamount of ethylene glycol by-products. This example illustrates thenecessity of adjusting the reaction mixture pH prior to esterification.

EXAMPLE 4 Esterification of 5-Sulfoisophthalic acid Sodium Salt (SIPA)at pH 1.0 without catalyst

A 500 mL round bottom flask was charged with 300 mL ethylene glycol and153.3 g of wet acidic cake containing 134.1 g of 5-sulfoisophthalic acidsodium salt. The pH of the slurry was determined to be 1.0. The reactionmixture was heated to reflux and the volatile components were distilledoff as the temperature rose. When the reaction reached 195° C., thereaction mixture turned black and started to stick to the stirrer. Inorder to prevent the reaction product from further sticking to thestirrer, the reaction was stopped after several hours and cooled to roomtemperature. The HPLC analysis of the resultant solid revealed less than5% product and many unidentified by-products. The distillate appeared tocontain water and ethylene glycol by-products. Again a poor yield wasobtained at low pH.

EXAMPLE 5 Esterification of 5-Sulfoisophthalic acid Sodium Salt (SIPA)at pH 1.6 without catalyst

A 500 mL round bottom flask was charged with 300 mL ethylene glycol and134.2 g of 5-sulfoisophthalic acid sodium salt obtained from acommercial vendor. The pH of the slurry was determined to be 1.6. Thereaction mixture was heated to reflux and water was distilled off fromthe reaction mixture as the temperature rose. The distillation continuedas the reaction was heated at 191° C. After 6 hours the product hadturned brownish black and was beginning to stick to the stirrer. Themixture was cooled to room temperature. The HPLC analysis of theresultant solidified product revealed less than 5% 5-sulfoisophthalicacid bisethylene glycol ester and many unidentified by-products. Thedistillate appeared to contain water and ethylene glycol by-products.Again a poor yield was obtained at low pH.

EXAMPLE 6 Esterification of 5-Sulfoisophthalic acid Sodium Salt (SIPA)at pH 2.3 without catalyst

A 1000 mL round bottom flask was charged with 600 mL of ethylene glycoland 307.2 g of wet acidic cake containing 268.2 g of 5-sulfoisophthalicacid sodium salt. The pH of the slurry was adjusted to 2.3 with 50%sodium hydroxide. The reaction mixture was heated to reflux and waterwas distilled from the reaction mixture as the temperature rose. Thetemperature was maintained at 195° C. for 6 hours. Residual sodiumsulfate was removed by filtration at 65° C. to give 804.2 g of solutioncontaining 302.7 g 5-sulfoisophthalic acid bisethylene glycol ester.

EXAMPLE 7 Esterification of 5-Sulfoisophthalic acid sodium salt (SIPA)at pH 2.7 with manganese acetate

A 500 mL round bottom flask was charged with 300 mL of ethylene glycoland 268.1 g of wet acidic cake containing 134.2 g of 5-sulfoisophthalicacid sodium salt. The pH of the slurry was adjusted to 2.7 with 50%sodium hydroxide and 0.6 g manganese acetate was added. The reactionmixture was heated to reflux and water was distilled from the reactionmixture as the temperature rose. The temperature was maintained at 195°C. for 2 hours. The residual sodium sulfate was filtered off at 65° C.to give 414.2 g of solution containing 137.3 g of 5-sulfoisophthalicacid bisethylene glycol ester.

EXAMPLE 8 Esterification of 5-Sulfoisophthalic acid Sodium Salt (SIPA)at pH 2.7 with sodium acetate

A 500 mL round bottom flask was charged with 300 mL ethylene glycol and153.3 g of wet acidic cake containing 134.2 g of 5-sulfoisophthalic acidsodium salt. The pH of the slurry was adjusted to 2.7 with 50% sodiumhydroxide and 0.64 g sodium acetate was added. The reaction mixture washeated to reflux and water was distilled from the reaction mixture asthe temperature rose. The temperature was maintained at 195° C. for 4hours. The residual sodium sulfate was filtered off at 65° C. to give440.4 g of solution containing 151.7 g 5-sulfoisophthalic acidbisethylene glycol ester.

EXAMPLE 9 Esterification of 5-Sulfoisopbthalic acid Sodium Salt (SIPA)at pH 5.0 with manganese acetate

A 500 mL round bottom flask was charged with 224 mL ethylene glycol and114.2 g of wet material containing 100 g of 5-sulfoisophthalic acidsodium salt The pH of the slurry was adjusted to 5.0 with 50% sodiumhydroxide and 0.4 g manganese acetate was added. The reaction mixturewas heated to reflux and water was distilled from the reaction mixtureas the temperature rose. The temperature was maintained at 195° C. for 2hours. The residual sodium sulfate was filter off at 65° C. to give301.0 g solution containing 66.4g 5-sulfoisophthalic acid bisethyleneglycol ester.

The foregoing examples are intended to illustrate the process of thisinvention and should not be construed to limit the scope of thisinvention. Many variations in the process of this invention will beobvious to one of ordinary skill in the art and such obvious variationsshould not be construed to be beyond the scope of the claimed invention.

We claim:
 1. A process for preparing a bis alkylene glycol ester ofsodium-5-sulfoisophthalate which comprises:(a) sulfonating isophthalicacid in the 5-ring position; (b) neutralizing the 5-sulfo ringsubstituent with an alkali metal neutralizing agent; (c) recovering thereaction products of said sulfonation and neutralizing steps; (d)dispersing said reaction products in an alkylene glycol and adjustingthe pH of said dispersion to about 1.7 to 5; (e) heating said dispersionto the form the bis alkylene glycol ester of said neutralized5-sulfoisophthalic acid and (f) separating said bis alkylene glycolester from insoluble reaction products.
 2. A process according to claim1 wherein said alkylene glycol is selected from ethylene and propyleneglycol.
 3. A process according to claim 1 wherein said pH is about 2 to3.
 4. A process according to claim 2 wherein said alkylene glycol isethylene glycol.
 5. A process according to claim 4 wherein said pH isabout 2 to
 3. 6. A process according to claim 1 wherein said bis glycolester is formed in the presence of an esterification catalyst.
 7. Aprocess according to claim 6 wherein said catalyst is sodium acetate. 8.A process according to claim 7 wherein said catalyst is manganeseacetate.